1. Technical Field
The invention relates to a device for scanning an information carrier having tracks around an axis, which device has means for transferring information from/to a scanning spot on the information carrier and has tangential displacement means for moving the scanning spot in a tangential direction and radial displacement means for moving the scanning spot in a radial direction, which device has control means for controlling the radial displacement means and has radial position measurement means for generating a radial position signal which is a measure of the instantaneous radial position of the scanning spot, which device has a scanning mode, in which the scanning spot follows a track, and a displacement mode, in which the scanning spot is moved in a radial direction and tracks are crossed.
The invention further relates to a method of operating such a device.
2. Related Art
A device of the type defined in the opening paragraph is known from U.S. Pat. No. 5,402,401. The device is suitable for reading and/or writing audio/video and/or file information from/onto a rotating information carrier such as a Compact Disc. In the scanning mode of the known device the radial error signal serves as a radial position signal. This radial position signal is used for centering the scanning spot on a track. In the displacement mode the scanning spot is moved in a radial direction in accordance with a predetermined velocity profile. On the one hand, a short duration of the displacement mode is desirable in order to obtain a short latency time between successive scanning modes and, consequently, an optimum efficiency of the information transfer. On the other hand, it is advantageous to effect the radial movement of the scanning spot comparatively slowly in order to limit the power consumption, noise production and wear in the displacement mode. A low power consumption is of particular importance for portable equipment.
It is an object of the invention to provide a device of the type defined in the opening paragraph, in which device the velocity of movement of the scanning spot can be limited without a consequent increase of the latency time. For this purpose, according to the invention, the device of the type defined in the opening paragraph is characterized in that the device further has tangential position measurement means for generating a tangential position signal which is indicative of the tangential position of the scanning spot, the control means causing the scanning spot to be moved in the radial direction in the displacement mode in a time interval with a radial displacement time which is a function of the difference between a desired tangential position and a current tangential position. This makes it possible to adapt the velocity of the movement in a radial direction to the duration of the time interval necessary to reach the desired tangential position. The invention is based on the recognition of the fact that it is not useful to move the scanning spot so rapidly that this scanning spot has already reached the desired radial position well before the desired tangential position is reached.
Since the scanning spot is moved to a desired radial position in a time interval longer than a minimum time interval, radial displacement requires less power and leads to a reduced noise production and a reduced wear. On the other hand, the rate at which information transfer from/to the information carrier is possible is not adversely affected because the radial displacement time is selected so as to allow for the difference between the desired tangential position and the current tangential position, as a result of which the desired radial and tangential positions are not reached any later than in the case that the radial movement is effected within the shortest possible length of time.
It is to be noted that from JP 07-176173 an apparatus is known in which a rotation signal which is indicative of a rotation of the information carrier is used for the correction of a counting signal. Thus, it is precluded that the rotation of an information carrier having spiral tracks leads to track counting errors.
In the device in accordance with the invention the control means provide a movement of the scanning spot over a similar radial distance in a comparatively long first radial displacement time and a comparatively short second radial displacement time for a comparatively small first difference and a comparatively large second difference in tangential positions.
In an embodiment of the device in accordance with the invention the control means include first control means for generating a signal which is indicative of the desired instantaneous position as a function of the time, second control means for generating an error signal which is indicative of a difference between the desired instantaneous position and the instantaneous position, and third control means for generating a control signal for the radial displacement means. This embodiment makes it possible to realize the radial movement within narrow tolerances within the radial movement time.
A variant of said embodiment is characterized in that the control means include fourth control means for generating a prediction signal which corresponds to an expected value of the control signal and include signal combination means for generating the control signal from the error signal and the prediction signal. This variant provides a very stable control of the radial displacement means in that only a small amplification of the error signal is necessary.
An embodiment is characterized in that the radial position measurement means have detection means coupled to the displacement means. The device may include, for example, a memory for storing a relationship between values of a detection signal generated by the detection means and numbers of tracks of the information carrier, so that it is known to which track a given value of the detection signal corresponds. Conversely, the track corresponding to a detection signal value can be calculated if the variations in track pitch are sufficiently small.
In an embodiment the radial position measurement means include track crossing counting means for generating a counting signal which is indicative of a number of tracks crossed, means for generating a tracking error signal which is indicative of the distance of the scanning spot from the nearest track, and means for generating a radial position signal from the counting signal and the tracking error signal. In this way, the radial position of the scanning spot can be determined accurately.
In the case of information carriers having continuous spiral tracks, in contradistinction to separate circular tracks, the counting signal is influenced by the rotation of the information carrier. For use with an information carrier having spiral tracks the device preferably has correction means for correcting the counting signal on the basis of the tangential position signal.
In accordance with the invention a method of operating a device for scanning an information carrier having tracks around an axis, in which method the information carrier is rotated about the axis, which method comprises a scanning mode in which information is transferred from/to a scanning spot on the information carrier and in which the scanning spot is moved along a track, and displacement mode in which the scanning spot is moved in a radial direction and in which tracks are crossed, is characterized in that in the displacement mode the scanning spot is moved in the radial direction in a time interval with a radial displacement time which is a function of the difference between a desired tangential position and a current tangential position.